Method and device for terminal calculating redistribution range in wireless communication system

ABSTRACT

Provided are a method for a terminal calculating a redistribution range in a wireless communication system, and a device supporting same. A terminal may receive redistribution factors from a network and calculate a redistribution range on the basis of a valid redistribution factor among the received redistribution factors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2016/005145, filed on May 16, 2016,which claims the benefit of U.S. Provisional Application No. 62/162,658filed on May 16, 2015, the contents of which are all hereby incorporatedby reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method for calculating, by a user equipment(UE), a redistribution range, and a device for supporting the same.

Related Art

3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) thatis an advancement of UMTS (Universal Mobile Telecommunication System) isbeing introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonalfrequency division multiple access) is used for downlink, and SC-FDMA(single carrier-frequency division multiple access) is used for uplink.The 3GPP LTE adopts MIMO (multiple input multiple output) having maximumfour antennas. Recently, a discussion of 3GPP LTE-A (LTE-Advanced) whichis the evolution of the 3GPP LTE is in progress.

Cellular is concept proposed to overcome a restriction of a serviceregion and a limitation of a frequency and subscriber capacity. This isa method of providing communication coverage by changing singlehigh-power base station to a plurality of low-power base stations. Thatis, a mobile communication service region is divided in unit of severalsmall cells so that different frequencies are assigned to adjacentcells, and two cells which are sufficiently spaced apart from each otherand thus have no interference occurrence use the same frequency band tospatially reuse a frequency.

Meanwhile, there may be a particularly high communication demand in aspecific region such as a hotspot inside a cell, and receptionsensitivity of radio waves may deteriorate in a specific region such asa cell edge or a coverage hole. With the advance of a wirelesscommunication technique, a small cell may be installed inside amacrocell for the purpose of enabling communication in the hotspot, thecell edge, and the coverage hole. A pico cell, a femto cell, amicrocell, or the like is a type of the small cell. The small cell maybe located inside or outside the macrocell. In this case, the small cellmay be located at a position where the macrocell does not reach, or maybe located indoors or at the office. Such a network may be called aheterogeneous network (HetNet). In this case, the heterogeneous networkdoes not have to use different radio access mechanisms. In aheterogeneous network environment, the macrocell is a relatively largecoverage cell, and the small cell such as the femtocell and the picocellis a small coverage cell. The macrocell and the small cell may serve todistribute the same traffic or transmit traffic of different QoS. In theheterogeneous network environment, coverage overlapping may occurbetween the plurality of macrocells and small cells.

SUMMARY OF THE INVENTION

When a plurality of frequencies is deployed in a heterogeneous network,a user equipment (UE) in a RRC_IDLE mode needs to perform an idle modechange based on load distribution information received from the networkin order to redistribute the UE in the RRC_IDLE mode. However, when theUE does not detect some frequencies in a frequency list included in asystem information block received from the network (that is, when the UEis located outside the coverage of some frequencies), the UE cannot beredistributed to some frequencies. Therefore, the present inventionproposes a method for calculating, by a UE, a redistribution range basedon a valid redistribution factor among redistribution factors receivedfrom a network, and a device for supporting the same.

According to one embodiment, there is provided a method for calculating,by a UE, a redistribution range in a wireless communication system. Themethod may include: receiving redistribution factors from a network; andcalculating a redistribution range based on a valid redistributionfactor among the received redistribution factors.

The redistribution range may be calculated as follows.

${{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = \frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {frequencies}\mspace{31mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}\;}$

The redistribution factors may be a redistribution probability value byfrequency for load balancing, and the valid redistribution factor may bea redistribution probability value of a frequency available for the UE.The method may further include performing, by the UE, a redistributionprocedure based on the calculated redistribution range. Theredistribution procedure may be performed between frequencies availablefor the UE.

The redistribution range may be calculated as follows.

${{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = \frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {cells}\mspace{14mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}\; {{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}$

The redistribution factors may be a redistribution probability value bycell for load balancing, and the valid redistribution factor may be aredistribution probability value of a cell available for the UE

The redistribution factors may be received via a system informationblock (SIB).

The method may further include receiving, by the UE, a frequency listfrom the network. The frequency list may be received via a SIB. Theredistribution range may be calculated when a frequency available forthe UE is different from a frequency included in the frequency list.

The UE may be in an RRC_IDLE mode.

According to another embodiment, there is provided a UE for calculatinga redistribution range in a wireless communication system. The UE mayinclude: a memory; a transceiver; and a processor to connect the memoryand the transceiver, wherein the processor may be configured to: controlthe transceiver to receive redistribution factors from a network; andcalculate a redistribution range based on a valid redistribution factoramong the received redistribution factors.

Load may be efficiently distributed in a heterogenenous networkenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem.

FIG. 3 shows a user plane of a radio interface protocol of an LTEsystem.

FIG. 4 shows a procedure in which UE that is initially powered onexperiences a cell selection process, registers it with a network, andthen performs cell reselection if necessary.

FIG. 5 shows an RRC connection establishment procedure.

FIG. 6 shows an RRC connection reconfiguration procedure.

FIG. 7 shows an RRC connection re-establishment procedure.

FIG. 8 shows an example of a heterogeneous network (HetNet).

FIG. 9 illustrates a method for a UE to calculate a redistribution rangeand to perform a redistribution procedure according to an embodiment ofthe present invention.

FIG. 10 illustrates a method for a UE to calculate a redistributionrange and to perform a redistribution procedure according to anembodiment of the present invention.

FIG. 11 is a block diagram illustrating a method for calculating, by aUE, a redistribution range according to an embodiment of the presentinvention.

FIG. 12 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced(LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routingtoward the gateway 30 during a radio resource control (RRC) activation,scheduling and transmitting of paging messages, scheduling andtransmitting of broadcast channel (BCH) information, dynamic allocationof resources to the UEs 10 in both UL and DL, configuration andprovisioning of eNB measurements, radio bearer control, radio admissioncontrol (RAC), and connection mobility control in LTE_ACTIVE state. Inthe EPC, and as noted above, gateway 30 may perform functions of pagingorigination, LTE_IDLE state management, ciphering of the user plane, SAEbearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem. FIG. 3 shows a user plane of a radio interface protocol of anLTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

A physical channel consists of a plurality of subframes in time domainand a plurality of subcarriers in frequency domain. One subframeconsists of a plurality of symbols in the time domain. One subframeconsists of a plurality of resource blocks (RBs). One RB consists of aplurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use specific subcarriers of specific symbols of acorresponding subframe for a PDCCH. For example, a first symbol of thesubframe may be used for the PDCCH. The PDCCH carries dynamic allocatedresources, such as a physical resource block (PRB) and modulation andcoding scheme (MCS). A transmission time interval (TTI) which is a unittime for data transmission may be equal to a length of one subframe. Thelength of one subframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom an upper layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). TheRRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The PDCP layer (terminated in the eNB on the network side) may performthe user plane functions such as header compression, integrityprotection, and ciphering.

Hereinafter, an RRC state of a UE and RRC connection procedure aredescribed.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC connected state and an RRC idlestate. When an RRC connection is established between the RRC layer ofthe UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE must transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell reselection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE can report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network can transmit and/or receive data to/from UE, thenetwork can control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC_IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

To manage mobility of the UE in the NAS layer, two states are defined,i.e., an EPS mobility management-REGISTERED (EMM-REGISTERED) state andan EMM-DEREGISTERED state. These two states apply to the UE and the MME.Initially, the UE is in the EMM-DEREGISTERED state. To access a network,the UE performs a process of registering to the network through aninitial attach procedure. If the attach procedure is successfullyperformed, the UE and the MME enter the EMM-REGISTERED state.

To manage a signaling connection between the UE and the EPC, two statesare defined, i.e., an EPS connection management (ECM)-IDLE state and anECM-CONNECTED state. These two states apply to the UE and the MME. Whenthe UE in the ECM-IDLE state establishes an RRC connection with theE-UTRAN, the UE enters the ECM-CONNECTED state. When the MME in theECM-IDLE state establishes an S1 connection with the E-UTRAN, the MMEenters the ECM-CONNECTED state. When the UE is in the ECM-IDLE state,the E-UTRAN does not have context information of the UE. Therefore, theUE in the ECM-IDLE state performs a UE-based mobility related proceduresuch as cell selection or reselection without having to receive acommand of the network. On the other hand, when the UE is in theECM-CONNECTED state, mobility of the UE is managed by the command of thenetwork. If a location of the UE in the ECM-IDLE state becomes differentfrom a location known to the network, the UE reports the location of theUE to the network through a tracking area update procedure.

FIG. 4 shows a procedure in which UE that is initially powered onexperiences a cell selection process, registers it with a network, andthen performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 shows an RRC connection establishment procedure.

The UE sends an RRC connection request message that requests RRCconnection to a network (S510). The network sends an RRC connectionestablishment message as a response to the RRC connection request(S520). After receiving the RRC connection establishment message, the UEenters RRC connected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 shows an RRC connection reconfiguration procedure.

An RRC connection reconfiguration is used to modify RRC connection. Thisis used to establish/modify/release RBs, perform handover, and setup/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

The following is a detailed description of a procedure of selecting acell by a UE.

When power is turned-on or the UE is located in a cell, the UE performsprocedures for receiving a service by selecting/reselecting a suitablequality cell.

A UE in an RRC idle state should prepare to receive a service throughthe cell by always selecting a suitable quality cell. For example, a UEwhere power is turned-on just before should select the suitable qualitycell to be registered in a network. If the UE in an RRC connection stateenters in an RRC idle state, the UE should selects a cell for stay inthe RRC idle state. In this way, a procedure of selecting a cellsatisfying a certain condition by the UE in order to be in a serviceidle state such as the RRC idle state refers to cell selection. Sincethe cell selection is performed in a state that a cell in the RRC idlestate is not currently determined, it is important to select the cell asrapid as possible. Accordingly, if the cell provides a wireless signalquality of a predetermined level or greater, although the cell does notprovide the best wireless signal quality, the cell may be selectedduring a cell selection procedure of the UE.

Hereinafter, a method and a procedure of selecting a cell by a UE in a3GPP LTE is described.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each UE set by a network may refer to a dedicated priority.If receiving the dedicated priority, the UE may receive a valid timeassociated with the dedicated priority together. If receiving thededicated priority, the UE starts a validity timer set as the receivedvalid time together therewith. While the valid timer is operated, the UEapplies the dedicated priority in the RRC idle mode. If the valid timeris expired, the UE discards the dedicated priority and again applies thecommon priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.

R _(S) =Q _(meas,s) +Q _(hyst) ,R _(n) =Q _(meas,n) −Q_(offset)  [Equation 1]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

FIG. 7 shows an RRC connection re-establishment procedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this procedure, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 shows an example of a heterogeneous network (HetNet).

Referring to FIG. 8, the heterogeneous network is a network in whichseveral types of cells co-exist. Many nodes are present in anoverlapping manner in the heterogeneous network, and a representativeexample thereof may include a picocell, a microcell, a femtocell, a homeeNB, or the like. Although a usage of the small cells is not limited, ingeneral, the picocell may be installed in a region having a high dataservice requirement, the femtocell may be installed in an indoor officeor home, and a radio relay may be installed for the purpose ofcompensating for coverage of the macrocell. In addition, the small cellsmay be divided into a closed subcarrier group (CSG) which can be used byonly a specific user, an open access which allows an access to a normaluser, and a hybrid access which uses the two methods in combination.

A plurality of frequencies may be deployed in the heterogeneous network.For example, macrocells having different frequencies may be deployed tooverlap, or small cells having different frequencies may be deployed tooverlap within a macrocell. When a plurality of frequencies may bedeployed in the heterogeneous network, the network needs to broadcast adistribution parameter relating to a carrier frequency (for example, aredistribution probability by frequency) via system information in orderto redistribute a UE in the RRC_IDLE mode. Then, the UE in the RRC_IDLEmode can perform an idle mode change according to the receiveddistribution parameter. For example, when the network broadcasts aredistribution probability by frequency through system information, theUE may generate a random uniform distribution value ranging between 0and 1 and may reselect a cell corresponding to the redistributionprobability. For a detailed description of cell reselection, supposethat the redistribution probability of a first macrocell (or firstfrequency) broadcasted by the network through the system information is0.1, the redistribution probability of a second macrocell (or a secondfrequency) is 0.2, the redistribution probability of a first small cell(or a third frequency) is 0.3, and the redistribution probability of asecond small cell (or a fourth frequency) is 0.4. When the random valuegenerated by the UE is 0.15, the UE can move to the second macrocell (orthe second frequency). When the random value generated by the UE is 0.7,the UE can move to the second small cell (or the fourth frequency). Thatis, for a successful idle mode change, the total sum of theredistribution probabilities by frequency needs to be 1.

However, when the UE in the RRC_IDLE mode is located outside the servicecoverage of some carrier frequencies in a carrier frequency listincluded in the system information, the total sum of the redistributionprobabilities by frequency broadcast from the network for the UE in theRRC_IDLE mode may not be 1. That is, in the foregoing embodiment, whenthe UE is located within the coverage of the first macrocell (or thefirst frequency), the second macrocell (or the second frequency), andthe first small cell (or the third frequency) but is located outside thecoverage of the second small cell (or the fourth frequency), the totalsum of the redistribution probabilities for the UE may be 0.6(=0.1+0.2+0.3). Therefore, the UE in the RRC_IDLE mode may not be ableto perform redistribution based on the redistribution probabilitiesbroadcast from the network. For example, when the UE in the RRC_IDLEmode is located outside the service coverage of some carrier frequenciesin a carrier frequency list included in the system information, the UEin the RRC_IDLE mode may not be redistributed to the correspondingfrequencies. Accordingly, in order to solve the foregoing problem, thepresent invention proposes a method for calculating, by a UE, aredistribution range and a device for supporting the same.

Hereinafter, a method for calculating, by a UE, a redistribution rangebased on a received distribution parameter according to an embodiment ofthe present invention will be described.

When a UE receives a distribution parameter from a network, the UE canperform a cell selection or cell reselection procedure using intendeddistribution statistics. In the present invention, the intendeddistribution statistics may be a set of redistribution factors. Theredistribution factors may be redistribution probability values byfrequency for load balancing that are received from the network.Alternatively, the redistribution factors may be redistributionprobability values by cell for load balancing that are received from thenetwork. When the UE cannot detect some carrier frequencies in a carrierfrequency list included in an SIB received from the network (that is,some carrier frequencies in the carrier frequency list included in theSIB are unavailable), the UE may need to consider new redistributionstatistics. The new redistribution statistics may be triggered by aparameter selected at a carrier frequency available for the UE. In thepresent invention, the new redistribution statistics may be a set ofnewly calculated redistribution ranges. A method for calculating aredistribution range may include the following steps.

(1) Step 1

Each UE may receive a SIB from a network. The UE may be a UE in theRRC_IDLE mode. The UE may classify frequencies included in a carrierfrequency list received through the SIB into a carrier frequency havinga distribution parameter and a carrier frequency having no distributionparameter. In the present invention, the distribution parameter may bereferred to as a redistribution factor. That is, the UE may classify thefrequencies included in the carrier frequency list received through theSIB into a carrier frequency having a redistribution factor and acarrier frequency having no redistribution factor.

(2) Step 2

The UE may detect a specific carrier frequency among carrier frequencieshaving a redistribution factor. The specific carrier frequency may be afrequency that can be detected/used by the UE as a result ofmeasurement. In the present invention, a redistribution factor of afrequency/cell that can be detected/used by the UE among redistributionfactors may be referred to as a valid redistribution factor.

Furthermore, the UE may calculate a redistribution range using the validredistribution factor. That is, the UE may calculate new redistributionstatistics using the valid redistribution factor.

The redistribution range may be defined by Equation 2.

$\begin{matrix}{{Wi} = \frac{Pi}{\sum\limits_{{all}\mspace{14mu} j\mspace{14mu} {in}\mspace{14mu} D}\; {Pj}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, i is a carrier frequency index in an index set for carrierfrequencies that can be detected/used by the UE; Pi is a receiveddistribution parameter associated with a carrier frequency index i; j isa parameter for normalization; and D is a set of carrier frequenciesthat can be detected/used by the UE.

That is, according to Equation 2, a redistribution factor (Pi)associated with an index i is divided by the sum of redistributionfactors of frequencies/cells that can be detected/used by the UE,thereby obtaining the redistribution range (Wi). That is, newredistribution statistics may be obtained.

The redistribution range may be defined by Equation 3 or 4.

$\begin{matrix}{{{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = \frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {frequencies}\mspace{31mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}\;}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \\{{{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = \frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {cells}\mspace{14mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}\; {{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

According to Equation 3 or Equation 4, similarly to Equation 2, a validredistribution factor is divided by the sum of all valid redistributionfactors, thereby obtaining a redistribution range.

(3) Step 3

The UE may perform a redistribution procedure between available carrierfrequencies based on the new redistribution statistics. That is, the UEmay perform a redistribution procedure between available carrierfrequencies based on the redistribution range calculated in Step 2.

FIG. 9 illustrates a method for a UE to calculate a redistribution rangeand to perform a redistribution procedure according to an embodiment ofthe present invention.

In FIG. 9(a), suppose that the UE is in the RRC_IDLE mode, is locatedwithin the coverage of a first macrocell, a second macrocell, and athird macrocell, but is located outside the coverage of a first smallcell. Suppose that the first macrocell uses a first frequency, thesecond macrocell uses a second frequency, the third macrocell uses athird frequency, and the first small cell uses a fourth frequency.

The first macrocell may broadcast intended redistribution statistics inorder to redistribute all idle UEs in the first macrocell to the firstmacrocell, the second macrocell, the third macrocell, or the first smallcell based on the intended distribution statistics. Suppose that theintended redistribution statistics are as follows.

-   -   Redistribution factor of first frequency: 0.1    -   Redistribution factor of second frequency: 0.2    -   Redistribution factor of third frequency: 0.3    -   Redistribution factor of fourth frequency: 0.4

(1) Step 1: Broadcast SIB Information

The first macrocell may broadcast SIB information. Therefore, the UE inthe RRC_IDLE mode located within the coverage of the first macrocell mayreceive a SIB. The SIB may include the following information.

-   -   Carrier frequency list: first frequency, second frequency, third        frequency, fourth frequency    -   Carrier frequency having redistribution factor (or distribution        parameter): first frequency (0.1), second frequency (0.2), third        frequency (0.3), fourth frequency (0.4)    -   Carrier frequency having no redistribution factor: None

Although the embodiment of FIG. 9(a) shows that there is no carrierfrequency having no redistribution factor, this is only an example.Instead, there may be a carrier having no redistribution factor.

(2) Step 2: Calculate Redistribution Range

In the embodiment of FIG. 9(a), since the UE is located within thecoverage of the first to third macrocells but is located outside thecoverage of the first small cell, a set (D) of frequencies that can beused/detected by the UE is {first frequency, second frequency, thirdfrequency}. Therefore, in the embodiment of FIG. 9(a), theredistribution factors of the first macrocell, the second macrocell, andthe third macrocell are valid redistribution factors. That is, D={firstfrequency, second frequency, third frequency} may be provided for theUEs. The UE cannot detect some frequencies (that is, the fourthfrequency) included in the carrier frequency list and thus may calculatea redistribution range. A redistribution range (or new redistributionstatistics) is calculated by Equation 2, Equation 3 or Equation 4 asfollows.

-   -   Redistribution range of first frequency: 0.1/(0.1+0.2+0.3)=0.17    -   Redistribution range of second frequency: 0.2/(0.1+0.2+0.3)=0.33    -   Redistribution range of third frequency: 0.3/(0.1+0.2+0.3)=0.5

(3) Step 3: Perform Redistribution Procedure

Each UE may select a random uniform distribution value ranging between 0and 1. Then, the UE may select a carrier frequency from {firstfrequency, second frequency, third frequency} associated with theselected random value. For example, suppose that a first UE selects avalue of 0.15, a second UE selects a value of 0.35, a third UE selects avalue of 0.59, and a fourth UE selects a value of 0.8. Referring to FIG.9(b), the first UE may (re)select a cell of the first frequency (thatis, the first macrocell), the second UE may (re)select a cell of thesecond frequency (that is, the second macrocell), the third UE may(re)select a cell of the third frequency (that is, the third macrocell),and the fourth UE may (re)select a cell of the third frequency (that is,the third macrocell).

FIG. 10 illustrates a method for a UE to calculate a redistributionrange and to perform a redistribution procedure according to anembodiment of the present invention.

In FIG. 10(a), suppose that the UE is in the RRC_IDLE mode and islocated within the coverage of a first macrocell, a second macrocell, athird macrocell, and a first small cell. Suppose that the firstmacrocell uses a first frequency, the second macrocell uses a secondfrequency, the third macrocell uses a third frequency, and the firstsmall cell uses a fourth frequency.

The first macrocell may broadcast intended redistribution statistics inorder to redistribute all idle UEs in the first macrocell to the firstmacrocell, the second macrocell, the third macrocell, or the first smallcell based on the intended distribution statistics. Suppose that theintended redistribution statistics are as follows.

-   -   Redistribution factor of first frequency: 0.1    -   Redistribution factor of second frequency: 0.2    -   Redistribution factor of third frequency: 0.3    -   Redistribution factor of fourth frequency: 0.4

(1) Step 1: Broadcast SIB Information

The first macrocell may broadcast SIB information. Therefore, the UE inthe RRC_IDLE mode located within the coverage of the first macrocell mayreceive a SIB. The SIB may include the following information.

-   -   Carrier frequency list: first frequency, second frequency, third        frequency, fourth frequency    -   Carrier frequency having redistribution factor (or distribution        parameter): first frequency (0.1), second frequency (0.2), third        frequency (0.3), fourth frequency (0.4)    -   Carrier frequency having no redistribution factor: None

(2) Step 2: Calculate Redistribution Range

In the embodiment of FIG. 10(a), since the UE is located within thecoverage of the first macrocell, the second macrocell, the thirdmacrocell, and the first small cell, a set (D) of frequencies that canbe used/detected by the UE is {first frequency, second frequency, thirdfrequency}. Therefore, in the embodiment of FIG. 10(a), theredistribution factors of the first macrocell, the second macrocell, thethird macrocell, and the first small cell are valid redistributionfactors. That is, D={first frequency, second frequency, third frequency,fourth frequency} may be provided for the UE. The UE detects allfrequencies included in the carrier frequency list and thus may notcalculate a redistribution range. That is, intended redistributionstatistics may be used as they are. Alternatively, although the UEdetects all frequencies included in the carrier frequency list, the UEmay calculate a redistribution range. In this case, the intendedredistribution statistics may be the same as new redistributionstatistics obtained by calculating a redistribution range.

(3) Step 3: Perform Redistribution Procedure

Each UE may select a random uniform distribution value ranging between 0and 1. Then, the UE may select a carrier frequency from {firstfrequency, second frequency, third frequency, fourth frequency}associated with the selected random value. For example, suppose that afirst UE selects a value of 0.15, a second UE selects a value of 0.35, athird UE selects a value of 0.59, and a fourth UE selects a value of0.8. Referring to FIG. 10(b), the first UE may (re)select a cell of thesecond frequency (that is, the second macrocell), the second UE may(re)select a cell of the third frequency (that is, the third macrocell),the third UE may (re)select a cell of the third frequency (that is, thethird macrocell), and the fourth UE may (re)select a cell of the fourthfrequency (that is, the first small cell).

FIG. 11 is a block diagram illustrating a method for calculating, by aUE, a redistribution range according to an embodiment of the presentinvention.

Referring to FIG. 11, the UE may receive redistribution factors from anetwork (S1110). The redistribution factors may be a redistributionprobability value by frequency for load balancing. Alternatively, theredistribution factors may be a redistribution probability value by cellfor load balancing. The redistribution factors may be received via asystem information block. The UE may be in the RRC_IDLE mode.

The UE may calculate a redistribution range based on a validredistribution factor among the received redistribution factors (S1120).The redistribution range may be calculated by Equation 3. In this case,the valid redistribution factor may be the redistribution probabilityvalue of a frequency available for the UE. Alternatively, theredistribution range may be calculated by Equation 4. In this case, thevalid redistribution factor may be the redistribution probability valueof a cell available for the UE.

The UE may receive a frequency list from the network. The frequency listmay be received via a system information block. In this case, theredistribution range may be calculated when a frequency available forthe UE is different from a frequency included in the frequency list.

The UE may perform a redistribution procedure based on the calculatedredistribution range. The redistribution procedure may be performedbetween frequencies available for the UE.

FIG. 12 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

A BS 1200 includes a processor 1201, a memory 1202 and a transceiver1203. The memory 1202 is connected to the processor 1201, and storesvarious information for driving the processor 1201. The transceiver 1203is connected to the processor 1201, and transmits and/or receives radiosignals. The processor 1201 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the basestation may be implemented by the processor 1201.

A UE 1210 includes a processor 1211, a memory 1212 and a transceiver1213. The memory 1212 is connected to the processor 1211, and storesvarious information for driving the processor 1211. The transceiver 1213is connected to the processor 1211, and transmits and/or receives radiosignals. The processor 1211 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the UE may beimplemented by the processor 1211.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The transceiver may include abase-band circuit for processing a wireless signal. When the embodimentis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory andmay be performed by the processor. The memory may be located inside oroutside the processor, and may be coupled to the processor by usingvarious well-known means.

Various methods based on the present specification have been describedby referring to drawings and reference numerals given in the drawings onthe basis of the aforementioned examples. Although each method describesmultiple steps or blocks in a specific order for convenience ofexplanation, the invention disclosed in the claims is not limited to theorder of the steps or blocks, and each step or block can be implementedin a different order, or can be performed simultaneously with othersteps or blocks. In addition, those ordinarily skilled in the art canknow that the invention is not limited to each of the steps or blocks,and at least one different step can be added or deleted withoutdeparting from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should benoted that those ordinarily skilled in the art know that all possiblecombinations of examples cannot be explained, and also know that variouscombinations can be derived from the technique of the presentspecification. Therefore, the protection scope of the invention shouldbe determined by combining various examples described in the detailedexplanation, without departing from the scope of the following claims.

What is claimed is:
 1. A method for calculating, by a user equipment(UE), a redistribution range in a wireless communication system, themethod comprising: receiving redistribution factors from a network; andcalculating a redistribution range based on a valid redistributionfactor among the received redistribution factors.
 2. The method of claim1, wherein the redistribution range is calculated by the followingequation:${{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = {\frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {frequencies}\mspace{14mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}\;}.}$3. The method of claim 2, wherein the redistribution factors are aredistribution probability value by frequency for load balancing, andthe valid redistribution factor is a redistribution probability value ofa frequency available for the UE.
 4. The method of claim 3, furthercomprising performing, by the UE, a redistribution procedure based onthe calculated redistribution range.
 5. The method of claim 4, whereinthe redistribution procedure is performed between frequencies availablefor the UE.
 6. The method of claim 1, wherein the redistribution rangeis calculated by the following equation:${{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = {\frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {cells}\mspace{14mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}\;}.}$7. The method of claim 6, wherein the redistribution factors are aredistribution probability value by cell for load balancing, and thevalid redistribution factor is a redistribution probability value of acell available for the UE.
 8. The method of claim 1, wherein theredistribution factors are received via a system information block(SIB).
 9. The method of claim 1, further comprising receiving, by theUE, a frequency list from the network.
 10. The method of claim 9,wherein the frequency list is received via a system information block(SIB).
 11. The method of claim 9, wherein the redistribution range iscalculated when a frequency available for the UE is different from afrequency comprised in the frequency list.
 12. The method of claim 1,wherein the UE is in an RRC_IDLE mode.
 13. A user equipment (UE) forcalculating a redistribution range in a wireless communication system,the UE comprising: a memory; a transceiver; and a processor to connectthe memory and the transceiver, wherein the processor is configured to:control the transceiver to receive redistribution factors from anetwork; and calculate a redistribution range based on a validredistribution factor among the received redistribution factors.
 14. TheUE of claim 13, wherein the redistribution range is calculated by thefollowing equation:${{Redistribution}\mspace{14mu} {{range}\lbrack i\rbrack}} = {\frac{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack i\rbrack}}{{\sum\limits_{j = 0}^{j = {{{Total}\mspace{14mu} {number}\mspace{14mu} {of}\mspace{14mu} {frequencies}\mspace{31mu} {with}\mspace{14mu} {valid}\mspace{14mu} {redistribution}\mspace{14mu} {factor}} - 1}}{{Valid}\mspace{14mu} {redistribution}\mspace{14mu} {{factor}\lbrack j\rbrack}}}\;}.}$15. The UE of claim 14, wherein the redistribution factors are aredistribution probability value by frequency for load balancing, andthe valid redistribution factor is a redistribution probability value ofa frequency available for the UE.